Use a separate admin page to show global stats, remove actions stat (#25062)

Before, Gitea shows the database table stats on the `admin dashboard`
page.

It has some problems:

* `count(*)` is quite heavy. If tables have many records, this blocks
loading the admin page blocks for a long time
* Some users had even reported issues that they can't visit their admin
page because this page causes blocking or `50x error (reverse proxy
timeout)`
* The `actions` stat is not useful. The table is simply too large. Does
it really matter if it contains 1,000,000 rows or 9,999,999 rows?
* The translation `admin.dashboard.statistic_info` is difficult to
maintain.

So, this PR uses a separate page to show the stats and removes the
`actions` stat.


![image](babf7c61-b93b-4a62-bfaa-22983636427e)

## ⚠️ BREAKING

The `actions` Prometheus metrics collector has been removed for the
reasons mentioned beforehand.
Please do not rely on its output anymore.

modules/metrics/collector.go

@@ -18,7 +18,6 @@ const namespace = "gitea_"
 // exposes gitea metrics for prometheus
 type Collector struct {
 	Accesses           *prometheus.Desc
-	Actions            *prometheus.Desc
 	Attachments        *prometheus.Desc
 	BuildInfo          *prometheus.Desc
 	Comments           *prometheus.Desc
@@ -56,11 +55,6 @@ func NewCollector() Collector {
 			"Number of Accesses",
 			nil, nil,
 		),
-		Actions: prometheus.NewDesc(
-			namespace+"actions",
-			"Number of Actions",
-			nil, nil,
-		),
 		Attachments: prometheus.NewDesc(
 			namespace+"attachments",
 			"Number of Attachments",
@@ -207,7 +201,6 @@ func NewCollector() Collector {
 // Describe returns all possible prometheus.Desc
 func (c Collector) Describe(ch chan<- *prometheus.Desc) {
 	ch <- c.Accesses
-	ch <- c.Actions
 	ch <- c.Attachments
 	ch <- c.BuildInfo
 	ch <- c.Comments
@@ -246,11 +239,6 @@ func (c Collector) Collect(ch chan<- prometheus.Metric) {
 		prometheus.GaugeValue,
 		float64(stats.Counter.Access),
 	)
-	ch <- prometheus.MustNewConstMetric(
-		c.Actions,
-		prometheus.GaugeValue,
-		float64(stats.Counter.Action),
-	)
 	ch <- prometheus.MustNewConstMetric(
 		c.Attachments,
 		prometheus.GaugeValue,